1. Field of the Invention
The present invention relates to a semiconductor device which is particularly suited for use as a surge voltage arrester.
2. Description of the Prior Art
Semiconductor devices used for electric circuits, such as thyristors, transistors, diodes, or integrated circuits composed of their combination, are often equipped with a surge voltage arrester to cope with surge voltages applied from external sources, and a voltage stabilizer element to cope with power-supply voltage fluctuations.
Devices used for these applications must have characteristics such that the resistances (or electric currents) are strikingly varied depending upon the applied voltages when various voltages are applied thereto. Particularly, such devices must have a large derivative of the V-I characteristics when voltages above a predetermined value are applied excellent reproduceability of the V-I characteristics, and a large durability against surge.
Conventional devices having the aforementioned characteristics include varistors that are made of inorganic compounds such as SiC, ZnO, and the like, Zener diodes, and semiconductor devices utilizing the avalanche phenomenon of planar PNP junction construction. A varistor made of an inorganic compound does not have so large a derivative or steep current increase in the V-I characteristics when a surge voltage is applied. Due to its large resistance, therefore, a varistor produces great amounts of heat. A varistor further has a large dispersion of current increase in the V-I characteristics. A Zener diode exhibits a more steep current rise in the V-I characteristics than the varistor. However, it generates large amounts of heat due to its dynamic impedance and provides less durability against surge. A semiconductive arrester of planar PNP junction construction utilizing the avalanche phenomenon has been disclosed, for example, in "A Silicon Voltage Limiter for Power Thyristor Circuits", Brown Boveri Rev., 9-72, pp. 476-482. The above semiconductive arrester is advantageous in that the dispersion of current increase in the V-I characteristics is less than that of varistors made of inorganic compounds. With respect to the current increase in the V-I characteristics and the generation of heat caused by the dynamic impedance, however, the semiconductive arrester has shortcomings similar to those of Zener diodes.
Because of the above-mentioned defects, the rated values of the devices protected by conventional surge voltage arresters have been selected to be considerably higher than the operation voltage of surge voltage arresters, resulting in increased manufacturing costs.
U.S. Pat. No. 3,427,509 describes a prior art device related to the present invention. This United States patent discloses a triggering diode of an N.sup.+ PN.sup.+ (or P.sup.+ NP.sup.+) construction, in which an N.sup.30 layer has a locally reduced thickness such that the breakdown voltage in the thin portion is decreased as compared with that of other portions, to thereby extinguish the negative resistance characteristics in one direction of the current, in order to obtain an asymmetrical V-I characteristic.